Geographic Variation in Dispersal Traits of Cakile edentula: Implications for the Evolution of Species' Range Limits

摘要

Dispersal is a key process that influences the geographic extent of species' ranges through colonization and gene flow. Models have commonly assumed that all populations within species have an equal dispersal kernel, but there is evidence that traits determining dispersal can vary due to genetic or environmental differences among populations. Geographic variation in dispersal traits has the potential to influence patterns of colonization and gene flow, however this has rarely been evaluated. I investigated dispersal trait variation in a Great Lakes beach plant, American sea rocket (Cakile edentula var. lacustris, Brassicaceae), and the potential consequences of that variation for the geographic range dynamics of the species. This species has a dimorphic fruit that has two dispersal strategies: a proximal fruit segment that stays attached to the plant, and a distal fruit segment that disperses short distances via wind and long distances via water. Specifically, I tested for 1) geographic variation in dispersal traits and potential environmental predictors from 30 sites across this species' geographic range; 2) the effects of dispersal kernel evolution and existing geographic patterns in dispersal on range dynamics and the response of dispersal to selection in a species' experiencing climate change; and 3) how differences in dispersal and phenotypic traits would influence performance of marginal and core populations during colonization into the range core. First, I found a relationship between many dispersal traits and latitude, which was significantly predicted by metrics of habitat quality and amount. A common garden experiment indicated that a proportion of this variation is due to genetic differences among sites. These results indicate that dispersal traits vary significantly, and even predictably, across the range of this species. Second, I developed an individual-based, spatially explicit model that incorporates latitudinal variation among populations in the dispersal kernel. Model results showed that dispersal kernel evolution and geographic variation in dispersal, where dispersal distance was longer at the range edges vs. the interior, allowed for the species to colonize new habitat more quickly under climate change. Third, I found that marginal populations phenotypic traits involved in local adaptation did not result in lower performance of marginal populations than range core residents in a field common garden, and lower dispersal ability did not occur in populations that had lower performance at the range. The results of this dissertation imply that dispersal variation could influence patterns of colonization and gene flow, and ultimately species range limits. This could impact the ability of species to respond to environmental change, such as climate change.